A Syrian rebel group says it has killed a Russian general working as an adviser to Syria's ministry of defence in an operation in the western Ghouta region on the outskirts of the capital Damascus.

A video statement from a group calling itself the "Hawks Special Operations Battalion ... a division of the Military Leadership of Damascus City and Province" gave the name of the general as Vladimir Petrovich Kochyev. The video, sent to Reuters, showed what the rebels said was a copy of his ID issued by the Syrian military.

There was no immediate comment from the Russian authorities.

"With the help of God, the Russian adviser for the minister of defence for military scientific affairs, General Vladimir Petrovich Kochyev, has been eliminated with his personal translator, Ahmad a-Ayyouq," Lieutenant Majid Sayyed Ahmad of the Hawks battalion said in the video.

"A number of Syrian army documents have been also seized, together with reports about the opposition and the Free Syrian Army, adding to the achievements on the road to the liberation of Syria from the Assad gang," said Ahmad, who gave his title as "commander of operations and special assignments".

The statement did not say when Kochyev was assassinated, but a senior opposition figure in contact with the group, who did not want to be named, said he was killed about two weeks ago.

He described Hawks battalion as a professional unit led by officers who had defected from the Syrian army.

The same group claimed responsibility for the assassination of four of President Bashar al-Assad's top lieutenants in Damascus last month.

Among the documents the video showed was a letter by the Syrian chief of staff, Fahed al-Freij, who is also the defence minister, asking the head of the Russian military advisers in Syria to extend Kochyev's term as adviser.

The video also showed a photo of Kochyev and another Russian officer in military uniforms posing with Freij. Its authenticity could not be independently verified.

Russia, which has an estimated several hundred military personnel in Syria, is one of the few countries still backing Assad diplomatically since a popular uprising against his rule erupted 17 months ago.

Meanwhile, Iran's foreign minister said some of the Iranians kidnapped by Syrian rebels last week are retired soldiers or Revolutionary Guards, Iranian media reported on Wednesday.

"Some of these beloved ones were on IRGC (Islamic Revolutionary Guard Corps) and military pensions ... and others were from other different departments," Ali Akbar Salehi said, according to Iran's student news agency ISNA. He denied they now had any military connection.

Syrian rebels said they kidnapped a busload of 48 Iranians on Saturday on suspicion of being military personnel. Tehran has said they were pilgrims visiting a Shi'ite shrine in Damascus.

Iran has steadfastly supported its ally Syrian President Bashar al-Assad in his attempts to crush opposition to his rule.

Salehi's comments were published after he visited Turkey on Tuesday to ask Ankara to use its links with Syria rebels to secure the Iranians' release.

Salehi said retired government employees from a number of agencies had signed up to make pilgrimages to Damascus after recent fighting in the Syrian capital had subsided.

"After some time in which pilgrims from Iran were not being dispatched to Syria...we took steps to send retired forces from various organizations," Iran's state news agency IRNA quoted Salehi as saying.

"Some retired individuals from the Guards and army were dispatched to Syria to make a pilgrimage."